ANODONTA — OIRCULATORY SYSTEM. 
177 
ridge on each side of the body, separated by the foot and known 
as tlie right and left ctenidia or hrancliia", according to tlieir position. 
They are each formed by a imir of trellised and vascnlar lanielhn, 
united at the base and each composed of a descending adaxial or 
jmmary lamella, which becomes 
Fig. 346. — Transverse section through the 
posterior region of A7iodonta cygnea^ im- 
mediately behind the foot, to show the arrange- 
ment of the organs and the coalition of the 
supra-branchial chambers of the inner gills, 
owing to the fusion of the secondary limbs 
of the right and left inner gills (after Howes). 
i.h. inner gill orbranchia; in. mantle lobes 
with marginal pallial muscles; o.hr. outer 
gill or branchia ; p.ad. posterior adductor 
muscle; r, rectum, showing typhlosole; sh. 
shell; sp.c. combined supra-branchial chamber 
of the inner gills ; v.g. visceral or parieto- 
splanchnic ganglia. 
tilameiitar spaces and bridge over 
of the cavity by interlamellar f 
cutely reflected to form an ascending- 
secondary limb, the distal margin 
of each external gill being fused to 
and vascularly continuous with 
the mantle lol)e of its side ; 
while the ascending or secondary 
limbs of the inner gills are similarly 
fused anteriorly with the visceral 
body wall, hut posteriorly, the 
secondary limbs of the right and left 
inner gills fuse together and form 
the partition or septum separating 
the branchial and cloacal cavities. 
The spaces or cavities between 
the primary and the secondary 
lamelhe of each gill are occupied 
by exuberant suh-filamentar out- 
gTowths, which obliterate the inter- 
and bind together the opposite sides 
ihrous or vascular junctions, which 
arise at regular intervals, coincident in position with or forming the 
larger vertical blood vessels. 
The vertical filaments, 
which were primitively the 
respiratory organs, have in 
Anodonta lost their original 
blood-carrying function and 
become comparatively firm 
and solid rods supporting the 
spong-y, lacunar and turges- 
cihle blood permeated tissue, 
within which definite blood 
channels are excavated and 
Fig. 347. — Fragment of the outer or primary limb 
of the inner gill <A Anodonta., torn from the connected 
inner or secondary lamella, to expose the constituent 
filaments and the fenestras, highly magnilied (after 
Holman Peck). 
b.7'. large vertical blood vessels ; f. fenestr.x* or 
water passages piercing the interlamellar lacunar out- 
growth, i.s. ; p.f. primitive vertical filaments ; t.f. 
transverse interfilamentar junctions. 
respiratory processes carried on. The interlamellar junctions in the 
M 
